diff --git a/ffi/ffi_handles.nim b/ffi/ffi_handles.nim
index c3e94e1..0052805 100644
--- a/ffi/ffi_handles.nim
+++ b/ffi/ffi_handles.nim
@@ -4,6 +4,8 @@
 ## FFI-thread-only access, so no locking.
 
 import std/tables
+import results
+import ./cbor_serial
 
 type
   FFIHandleRoot* = ref object of RootObj
@@ -36,20 +38,29 @@ proc register*(
 
 proc lookup*(
     reg: var FFIHandleRegistry, handle: uint64, typeName: string
-): FFIHandleRoot =
-  ## Live ref for `handle`, or nil if absent or registered under another type.
+): Result[FFIHandleRoot, string] =
+  ## Live ref for `handle`; err if absent or registered under another type.
   let entry = reg.byHandle.getOrDefault(handle)
-  if entry.obj.isNil() or entry.typeName != typeName:
-    return nil
-  entry.obj
+  if entry.obj.isNil():
+    return err("no ffiHandle with id " & $handle)
+  if entry.typeName != typeName:
+    return err(
+      "ffiHandle " & $handle & " has type '" & entry.typeName & "', expected '" &
+        typeName & "'"
+    )
+  ok(entry.obj)
 
 proc release*(reg: var FFIHandleRegistry, handle: uint64): bool {.discardable.} =
   ## Drops the entry; true iff it existed.
   if not reg.byHandle.hasKey(handle):
     return false
   reg.byHandle.del(handle)
-  true
+  return true
 
 proc releaseAll*(reg: var FFIHandleRegistry) =
   ## Drops every entry. Must run on the FFI thread that allocated the refs.
   reg.byHandle.clear()
+
+proc encodeHandle*(id: uint64): seq[byte] =
+  ## Wire-form bytes for a handle id. Single ABI seam for future format changes.
+  cborEncode(id)
diff --git a/ffi/internal/ffi_macro.nim b/ffi/internal/ffi_macro.nim
index 80f5197..7ec84fe 100644
--- a/ffi/internal/ffi_macro.nim
+++ b/ffi/internal/ffi_macro.nim
@@ -123,12 +123,11 @@ proc unpackHandleField*(
 ): NimNode =
   ## Reconstitutes a handle param from its wire `uint64` via the ctx registry,
   ## returning `RET_ERR` (Result.err) on a stale/forged/wrong-type id.
-  let errMsg = "invalid or stale ffiHandle for parameter '" & $fieldIdent & "'"
+  let errPrefix = "ffiHandle for parameter '" & $fieldIdent & "': "
   quote:
     let `fieldIdent` = block:
-      let ffiH = `ctxIdent`[].handles.lookup(`decodedIdent`.`fieldIdent`, $`userType`)
-      if ffiH.isNil():
-        return err(`errMsg`)
+      let ffiH = `ctxIdent`[].handles.lookup(`decodedIdent`.`fieldIdent`, $`userType`).valueOr:
+        return err(`errPrefix` & error)
       cast[`userType`](ffiH)
 
 proc cExportedParams(ctxType: NimNode): seq[NimNode] =
@@ -431,7 +430,7 @@ proc addNewRequestToRegistry(reqTypeName, reqHandler: NimNode): NimNode =
       return ok(newSeq[byte]())
     elif typeof(`typedResIdent`.value) is FFIHandleRoot:
       return ok(
-        cborEncode(
+        encodeHandle(
           `handlerCtxIdent`[].handles.register(
             `typedResIdent`.value, $typeof(`typedResIdent`.value)
           )